This invention relates to a sensing apparatus for detecting and measuring a physical quantity by utilizing light and photodetectors.
A measurement system utilizing light is preferable for measuring physical quantities since a large amount of information can be transmitted by light and is not easily subject to distortion. Measurement systems utilizing sources other than light (e.g., electricity) can be easily affect by various factors such as magnetic induction or grounding abnormalities.
Various types of sensors have been proposed for use in a measurement system incorporating optical fibers as a transmission line. It is important that signal variation or signal loss along the optical fiber transmission path and light source variations do not substantially affect the detected output signal.
For this purpose, two conventional methods have been used. One method utilizes a common light source for two different transmission paths. The first path senses the physical quantity within a detecting area while the second path is not affected by the physical quantity. The outputs of both paths are then compared to measure the physical quantity.
Another method utilizes two sources having a common path. Moreover, both paths extend through the detecting area for detecting the physical quantity. The sources produce a light having two different wavelengths; thus, two detecting signals are produced whose ratio can be used to calculate the physical quantity. That is, since the variation of the physical quantity is known separately for each wavelength, a graph can be drawn of the variation of the physical quantity versus the ratio of the two wavelengths. Upon measuring the ratio by the second method, the physical quantity can be determined from the graph.
These conventional methods, however, have certain disadvantages. While the first method corrects for variations in source intensity and changes in the ambient condition by cancelling the disturbances along both paths, other disturbances are not compensated. For example, in the first method, the first path through the transducer portion (i.e., detecting area) has a different characteristic caused by the physical quantity. Thus, both paths do not always receive an equal disturbance caused by the physical quantity. Therefore, the effect of a disturbances is not completely compensated.
In the latter method, the disturbance caused by the physical quantity will be compensated. However, the use of different wavelengths will cause different intensities produced at the source; further, the characteristics of the transmission path will vary from wavelength to wavelength. As a result, the accuracy of the measurement system will be adversely affected. Even if the effect of the disturbances can be minimally suppressed, it is necessary to keep the output intensity ratio of the two sources constant or to monitor these respective output intensities. In that event, it is necessary to use expensive and sensitive optical equipment, such as light separators or light multiplexors. As a result, the reliability of the system will be decreased while the system cost will be adversely increased.